The specific aims of this application are: (1) to determine the numbers, distribution, and types of neurons and synaptic connections present in regions of developing buds and gonads of Hydra; and (2) to characterize morphological differences among sensory cells in this simple nervous system. The methodology will include scanning (SEM), ordinary transmission (TEM), and high voltage (HVEM) electron microscopy combined with immunocytochemical and/or immunofluorescent techniques for information on neuropeptides. The long-term objectives are: (1) to characterize the complete neuronal architecture of a simple organism and trace its synaptic connections; and (2) to develop a logical hypothesis to explain the probable evolution of simple sensory cell features, neuroendocrine characteristics, and synaptic structures into more complex systems found in higher organisms including man. The disciplines involved include cell and developmental biology, invertebrate neurobiology and evolution. The health-relatedness of the project is based on the common principles that underlie the structure, function and development of all neurons and their synaptic associations. Furthermore, the technical approaches developed in this attempt to reconstruct a simple nervous system can ultimately be modified for application to a study of more complex systems including the brain of man.